Re: [Caml-list] still silly issues on polymorphic types

[Walter Cazzola <cazzola@dico.unimi.it>]

On Tue, 27 Sep 2011, Christophe Papazian wrote:

>> uhm, I have a couple of questions I don't understand on your
>> explanation:
>> - why the most general type is int->'a and not 'a -> 'b?

> Because if I need a function of type int -> 'a I can give a function of type 
> 'a -> 'b.
> But if I need a function of type 'a -> 'b I can not use a function of type 
> int -> 'a.
> So int -> 'a is more general and 'a -> 'b is more specific.

> General rule : if a < b then (a->c) > (b->c)

probably this is a question whose answer will be RTFM but I'll try to
get your mercy (;-)): what define that a is lesser than b? is there a
table of the precendece among types?

>> - does this mean that I can't have a general type that could be matched
>>  by 'a -> 'a list -> 'a list  and int -> int -> int ?

> if you want to get a supertype of a -> b and c -> d you need to
> find a subtype of a and c AND find a supertype of b and d.

> So for example for ('a -> ( 'a list -> 'a list)) and (int -> (int -> int))
> you need
> - a subtype of 'a and int : it's int. you have one.
> - a supertype of ('a list -> 'a list) and (int -> int) (reapply the same 
> procedure)
>     -a subtype of 'a list and int : there is no such thing in ocaml, so 
> there is no supertype.

ok I see

> So to answer your question, you can't. But as Jacques Garrigue said, you 
> don't need either, and you can use abstract type and "with type"
> construction.

uhm, as I replied to Jacques I tried but the error persists probably I
don't know enough OCaML to do that.

thanks a lot
Walter

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